Highly brominated aryl alkyl ethers have utility as fire retardants for organic, polymeric resinous materials. See, for example, U.S. Pat. Nos. 3,658,634; 3,717,609; 3,808,171; 4,016,137; 4,016,139; 4,032,507; and 4,032,508; and Japanese Pat. No. 14,500 (72). In common with other highly halogenated molecules, such as, for example, many of those disclosed in U.S. Pat. Nos. 3,403,306 and 3,418,263, many of the highly brominated aryl alkyl ethers, when compounded in plastics and exposed to sunlight, are subject to discoloration. With many, a gradual yellowing occurs that is undesirable and that is frequently masked by the use of dark brown pigments or even paint. There is a need for light stable fire retardant additives for plastics. As a general rule, the more bromination that can be achieved in the aryl group, the more effective the compound is as a fire retardant. Heretofore it has been difficult to brominate to as much as three bromine atoms per phenyl group in bisphenoxyalkanes, and most difficult if not impossible completely to brominate the phenyl group in bisphenoxyalkanes, without cleaving one or both of the phenoxy-to-alkylene linkages.
One way of making the highly brominated bisphenoxyalkane compounds, that is disclosed in U.S. Pat. No. 4,016,137, for example, involves reacting a brominated phenol with an alkylene halide. This is itself an acceptable synthesis step, but there is a great practical difficulty in producing a brominated phenol that contains four or five bromines per molecule. To date the production of such highly brominated phenols has been so difficult as to prevent their use for the further step of producing the brominated bisphenoxyalkanes on a practical, commercial basis.
To produce highly brominated aromatic compounds, especially fully brominated compounds, rigorous reaction conditions have been considered necessary, i.e., high temperatures, and large amounts of active catalyst. Under these conditions, if elemental bromine is used as the brominating agent, hydrogen bromide is produced as a by-product, and this degrades the product formed. In addition, it is wasteful of bromine. Similarly, under such conditions, the use of Lewis acid catalysts for the bromination also resulted in degradation of the product. When the aromatic compound being brominated is a bisphenoxyalkane, cleavage of the phenoxy-to-alkylene linkage occurs. Hydrogen bromide causes the formation of phenols, R. L. Burwell, Chem. Rev. 54, at 630 (1954), and Lewis acid catalysts promote the formation of phenol salts, idem. at 654. Diphenyl ether, however, is not cleaved by strong acids, idem. at 628.
Thus brominated anisoles when heated with hydrogen bromide in acetic acid for two hours on a steam bath decompose to an extent of 21-85%. D. M. Birosel, J. Am. Chem. Soc., 53, 1408 (1931). When anisole is heated for two hours at 100.degree. C. with aluminum chloride, methyl chloride evolves and leaves behind Cl.sub.2 AlOC.sub.6 H.sub.5. G. Baddeley, J. Chem. Soc. 1944, 330. When the bromination of anisole is catalyzed by aluminum chloride, Bonneaud, Bull. Soc. Chim., Fr. (4)7, 776 (1910), or iodine, A. I. Hashem, J. Appl. Chem. Biotechnol, 24, 59 (1974), only pentabromophenol is recovered. Thus anisoles cannot survive drastic bromination conditions. Under mild conditions a maximum of three bromine atoms can be introduced per aromatic ring. The practicality of introducing even three bromine atoms per aromatic ring under non-degrading reported conditions is questionable.
The bromination of anisole with one equivalent of bromine is chloroform, at room temperature, is reported to give an 80% yield of monobromoanisole. Grignard, Bellet and Courtot, Ann. Chim. 4, 28 (1915). The preparation of dibromoanisole from anisole, and of tribromoanisole from dibromoanisole, in carbon tetrachloride, is also reported. Kohn and Sussman, Monatsh. Chem. 46, 575 (1925). "Several days" reaction time is reported for the preparation of several tribromophenyl alkyl ethers. Yields are not reported. Baiford and Birosel, J. Am. Chem. Soc., 51, 1776 (1929). The bromination of anisole with three equivalents of bromine yields 1.5% tribromoanisole. Similarly, the bromination of monobromoanisole with two equivalents of bromine in chloroform is reported to give only small quantities of tribromoanisole, and tribromoanisole is reported not to brominate further. D. M. Birosel, Univ. Phillipines Natural Applied Sci. Bull. 1, 145 (1931).
The monobromination of anisole with a solution containing chlorine and bromine is one of the examples listed in U.S. Pat. No. 2,607,802 (1951).
1,2-Bis(4-bromophenoxy)ethane has been prepared in 50-70% yield by reaction of bromine in acetic acid. A. C. Cope, J. Am. Chem. Soc., 57, 572 (1935). A tetrabrominated derivative of 1,2-diphenoxyethane is reported by bromination in chloroform. Lippman, C. R., Acad. Sci., 68, 1269 (1869).
Bis(tribromophenoxy)-ethane and bis(tetrabromophenoxy)ethane have been prepared from brominated phenols and ethylene dibromide, by utilizing the brominated phenol as a reactant. M. Kohn and A. Fink, Monatsh. Chem., 44, 194 (1924). Dow has been granted a patent for the polybromination, with bromine chloride, of aromatic compounds other than aryl alkyl ethers, in a closed system. U.S. Pat. No. 3,845,146 (1974).
Accordingly, a dilemma faces one seeking the production of highly brominated aryl alkyl ethers. If conditions conducive to substantial bromination are used, such as strong Lewis acids and relatively high temperatures, then degradation and cleavage of the phenoxy-to-alkylene linkage results. If, to avoid this, milder conditions are used, such as no catalyst or a weak Lewis acid catalyst and relatively low temperatures, then an unsatisfactory, low level of bromination results.
It would, therefore, advance the art to be able to produce highly brominated bisphenoxyalkanes, especially at relatively high yields, in a manner which does not degrade the product nor cleave the phenoxy-to-alkylene linkage.